Data transmission rates in a fiber optic communication system that uses optical fibers to transmit signals are seeing even greater improvements. In a fiber optic communication system, an optical fiber that is a transmission path or transmission line has chromatic dispersion as one of its characteristics, and the waveform distortion produced in optical signals by this chromatic dispersion is a factor that limits transmission rate and transmission distance. Accordingly, there is a need for technology for accurately measuring chromatic dispersion in the optical fiber that is the transmission path and then adjusting in accordance with the measurement result to make the chromatic dispersion substantially zero. Adjustment techniques for making the chromatic dispersion substantially zero are known as, for example, equalization or dispersion compensation. In the following explanation, chromatic dispersion is simply referred to as “CD.”
In a fiber optic communication system, because the two ends of a transmission path are typically in separated locations and the chromatic dispersion of the optical fiber varies according to the temperature and external pressure, the measurement and adjustment of CD must be carried out on the far end, i.e., the receiving end of an optical signal, during operation of the system.
As a first example of the related art for meeting these demands, a PM-AM conversion method is used as a measurement method on the far end and a monitor light of a different wavelength than the signal transmission is used to detect CD of the transmission path during system operation. The PM-AM conversion method uses the principle that upon transmission of a phase-modulated monitor light, the monitor light that has undergone phase-modulation (PM) is converted to amplitude modulation (AM) under the influence of CD. The first example of the related art is shown in, for example, Kuwahara Shoichiro, et al., “Adaptive dispersion equalization with the detection of dispersion fluctuation using the PM-AM conversion,” Abstract for the Annual Meeting 1998 of Communications Society: Institute of Electronics, Information and Communication Engineers (IEICE), pp. 417 (1998) [NPL1]. The first example of the related art is here described based on the paper by Kuwahara, et al.
FIG. 1 shows the system shown by Kuwahara et al. In the transmission end of this system, a signal light from optical transmitter (TX) 1400 to which a data signal having a high bit rate is applied and a monitor light are multiplexed by optical coupler (CPL) 1412, and the combined light is transmitted to transmission path 1404 such as an optical fiber. Laser light source 1401 of a wavelength that differs from that of the signal light, sine wave generator (SINE GEN) 1402, and phase modulator (PHASE MOD) 1403 are provided to generate the monitor light. In phase modulator 1403, the monitor light is generated by subjecting the light from laser light source 1401 to phase modulation by the sine wave signal from sine wave generator 1402.
The signal light and the monitor light that is of a different wavelength than the signal light are propagated on transmission path 1404. These two light beams propagated on transmission path 1404 come into wavelength demultiplexer (CPL) 1405 at a reception end. Wavelength demultiplexer 1405 separates the received light into the signal light and monitor light. Of these, the signal light is received into optical receiver (RX) 1406, whereby the data signal is reproduced from the signal light. On the other hand, the monitor light is propagated through chromatic dispersion compensator (CD COMP) 1407 and then comes into photodetector (PD) 1408. Photodetector 1408 carries out square-law detection of the monitor light, whereby the output of photodetector 1408 is proportional to the amplitude modulation component in the monitor light. Average measurement circuit (AVG) 1411 and band-pass filter (BPF) 1410 are provided at the output of photodetector 1408, whereby the average level of the detection signal of photodetector 1408 and the intensity of the frequency component of the sine wave signal used in the phase modulation on the transmission side are found. Control circuit 1409 finds the value of CD on transmission path 1404 from the ratio of the average level of the detection signal and the intensity of the frequency component of the sine wave signal and generates a control signal that becomes feedback to the transmission side.
In this system, adjustment by a known method is carried out before operation of the system such that residual CD at the wavelength of the signal light becomes zero. At this time, CD at the monitor light wavelength that differs from the wavelength of the signal light typically does not become zero due to the wavelength dependence of CD. Consequently, in order to also set CD at the monitor light wavelength to zero, the amount of compensation is adjusted in CD compensator 1407 with respect to monitor light that has been separated from the signal light.
When adjustment is thus carried out before operation to set CD relating to the signal light and monitor light to zero and CD of transmission path 1404 diverges from zero during operation, the monitor signal that is undergoing phase modulation is converted to intensity modulation by CD, whereby a frequency component of the sine-wave signal used in phase modulation appears in the square-law detection output of photodetector 1408 on the receiving side. Control circuit 1409 thereupon judges whether CD relating to the monitor light has diverged from zero based on the ratio of the average level of the detection signal and the intensity of the frequency component of the sine-wave signal. Upon detecting that CD has diverged from zero, control circuit 1409 transmits a control signal to the transmitting side to initiate control for changing the wavelength of the monitor light such that the CD detected at photodetector 1408 relating to monitor light is made zero.
When the wavelength of the monitor light is changed and the sine-wave signal frequency component in the detection signal at photodetector 1408 becomes zero, the transmission path CD in the monitor light also becomes zero, whereby the wavelength control of the monitor light is halted. The wavelength of the signal light is then shifted by the amount that the wavelength of the monitor light at this time has been already shifted. In this way, CD relating to the signal light can again be set to zero. Thus, by means of the first example of related art, detecting shift of CD of a monitor light from zero enables control such that the CD of the signal light wavelength becomes zero.
As a second example of the related art relating to the present invention, JP-A-2000-346748 [PL1] discloses the monitoring of a CD value by, when transmitting a data signal by means of wavelength multiplexing that uses two different wavelengths, superposing a signal for CD measurement on the optical signal that follows multiplexing and then detecting this signal for measurement on the reception end. In this second example of the related art, an intensity modulation signal is used as an in-service signal used in data transmission and this intensity-modulated signal is wavelength-multiplexed. Then, using a sine-wave signal that has been phase modulated by a pseudo-random code as a CD detection signal, the superposition of the signal is carried out by applying minute intensity-modulation that is driven by the CD detection signal to the optical signal that follows wavelength multiplexing. When there is chromatic dispersion, a difference occurs between the two wavelengths used in signal transmission in the times of arrival at the receiving side of the CD detection signal, whereby the CD detection signals are demodulated at each of the two wavelengths on the receiving side and the time difference in the demodulated codes is detected to enable detection of the CD. Because this technique employs pseudo-random codes, it has the advantage that detection accuracy does not decrease even in cases in which the superposition level of the CD detection signal cannot be made large.
As the third example of the related art of the present invention, JP-A-2003-134047 [PL2] discloses the measurement of wavelength dependency of transmission delay of a transmission path and finding chromatic dispersion in an optical transmission system that carries out wavelength multiplexing, by detecting frames belonging to the data signal of each wavelength channel either constantly or at short repeating times and then carrying out a relative comparison of the frame phases for each wavelength channel.